Depolarizer for rechargeable cells



3,024,297 DEPOLARIZER FOR RECHARGEABLE CELLS Lewis F. Urry, Parma, Ohio, assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Nov. 12, 1959, Ser. No. 852,201 Claims. (Cl. 136139) This invention relates to rechargeable alkaline galvanic cells and it more particularly relates to an improved cathode-depolarizer for such cells.

In order to produce electric power by galvanic action, a cell system comprising an anode, a cathode-depolarizer, and an electrolyte must be chemically reacted. In this process, the materials used for these elements become exhausted to an extent that upon discharge of the cell, the anode has become oxidized and the cathode-depolarizer has become reduced to totally diiferent chemical substances from those which initially made up the cell. In certain galvanic systems, it is possible to reactivate the cell elements by passing current through the cell. This current effectively reverses the electricity producing process, thus recombining the elements of the cell into a charged state such that the cell may again be discharged to produce electric power by galvanic action. It is obvious that cells of this type are very advantageous for certain applications. 'Some of the more common uses to which rechargeable cells are put are automobile starting systems and submarine power plants.

Certain cathode-depolarizers which are well known to the primary galvanic cell art have come to the attention of people working with alkaline secondary (rechargeable) cells because of their availability and relatively moderate price. It has been suggested that either mercuric oxide or manganese dioxide would have advantages to ofier the recharageable cell field. Mercuric oxide may be discharged at high current drains while yielding a high output per unit volume. Another important characteristic of this material is that its voltage against time discharge curve is relatively flat. Unfortunately, these good properties of mercuric oxide are oifset by the fact that upon discharge, metallic mercury is formed and this material coalesces. When it is sought to recharge the mercury by electrolytic oxidation, only the outside surface of the mercury can be oxidized because the oxide formed effectively shields the remainder of the mercury from further oxidation. In addition to this, mercury charges at a relatively constant potential, until the charging is complete, at which time the cell voltage rises substantially instantaneously to the decomposition potential of the electrolyte of the cell being charged. This steep rise makes it practically impossible to put a voltage cutoff sensing device in the system which is capable of stopping the charging operation before the electrolyte decomposition potential is reached.

Manganese dioxide has been considered as a cathodedepolarizer for rechargeable alkaline galvanic cells because of its low cost, good depolarizing action, and ready availability. This material, however, shown a tendency to discharge at decreasing voltage over a period of time. Also, while a spent manganese dioxide electrode is rechargeable by the passage of current therethrough, this rechargeability is lost when it is discharged beyond a certain critical oxidation level. Thus, this material cannot be effectively recharged after a deep discharge.

It is therefore the principal object of this invention to provide a cathode-depolarizer for rechargeable alkaline galvanic cells which operates at high currents with a high output per unit volume.

It is another object of this invention to provide such a cathode-depolarizer which exhibits a relatively flat discharge curve under load.

It is a further object of this invention to provide such 3,024,297 Patented Mar. 6, 1962 a cathode-depolarizer which exhibits a charging curve such that a voltage cutoff device may be used to maintain the charging voltage below a level which is destructive to other cell elements.

These objects are fulfilled by this invention which comprises a cathode-depolarizer mix for rechargeable alkaline galvanic cells which comprises 13 percent to 95 percent by weight of manganese dioxide and 5 percent to 87 percent by weight mercuric oxide.

It is necessary in the practice of this invention that the proportion of mercuric oxide in the mixture be less than that quantity which when reduced to mercury Will coalesce to form an agglomorate mass. This maximum has been determined to be 87 percent by weight or percent by volume of the oxide. The remainder of the mix may be either manganese dioxide alone or manganese dioxide mixed with carbon. The range of proportions which have been found useful for the practice of this invention are 5 percent to 87 percent by weight, or 3 per cent to 75 percent by volume, mercuric oxide; and 13 percent to percent by weight, or 25 percent to 97 percent by volume, manganese dioxide. The preferred range of proportions is 52 percent to 87 percent by weight, or 33 percent to 75 percent by volume, mercuric oxide; and 13 percent to 48 percent by weight, or 25 percent to 67 percent by volume, manganese dioxide. While it has been stated that a conductive material, such as carbon, may be added to the cathode-dcpolarizer mix as described, it is to be understood that one of the features of this invention is that the mixture described is sufficiently con- .ductive of its own accord to sustain the galvanic action of the cell without the inclusion of an additional conductive material.

It has been found expedient toprepare the mixed depolarizer by coating a particle of mercuric oxide with a thin shell of manganese dioxide. This procedure may be carried out by adding finely-powdered mercuric oxide to a clear aqueous solution of manganese nitrate. The mercuric oxide, being insoluble in the manganese nitrate solution, forms a slurry wherein each particle of the om'de is coated with a thin film of solution. After thorough mixing, the slurry is dried until a damp mass is obtained whereupon the mass is broken up into pellets. These pellets are baked causing the thin film of manganese nitrate to decompose into manganese dioxide and adhere to the mercuric oxide particles. Upon decomposition, the manganese nitrate also forms volatile nitric oxide which passes out of the system and may be collected. If desired, the pellets of manganese dioxide-coated mercuric oxide particles may be crushed to a powder, suitably smaller than 0.0029 inch and used directly as a cathode-depolarizer in a rechargeable alkaline galvanic cell.

The thin layer of manganese dioxide formed by the above-described method serves to keep each individual mercuric oxide particle from physical contact with other like particles. This manganese dioxide layer, when in the reduced oxide form particularly, is hard, solid, electrolyte insoluble, porous, and semi-conductive. Because of these properties, electrolytic and electric contact with the mercuric oxide is permitted. Also, and probably more important, is the fact that the reduced manganese dioxide layer prevents the mercury, formed by the discharge of the mercuric oxide, from physically contacting other similar pools of mercury and coalescing to a non-rechargeable mass. Therefore, both oxides contribute as cathodedepolarizer material, and both materials are rendered rechargeable because of their physical relationship brought about by this process. In addition to these physical advantages, the voltage charging curve for the combined oxides is flat until all the mercury is charged to mercuric oxide and then rises rapidly but not instantaneously while the reduced oxides of manganese are being charged.

I into particles less than 0.003 inchin diameter. and stored I I I I I I I I I I f I Suchbehaviori makes it possible tozelectrically insure the I f I non-destructive charging ot'a cell containing these materi- I I for future use in rechargeable alkaline cells. I I I I I I I I I I I als'by the inclusion of a voltage cutoff deviceto stop the I I I i cathoderdepolarizer m'ix; made; according to this inyem I I I I II I i charging current when a certain predeterminedvoltageis: I I tion has been incorporatedin standard rechargeable alka: I I I I I reached; I This mixture permits construction I of cells I 5 I line cells and testedto determine the characteristics of its I I I I I I I which are capable of withstanding heavy discharge drain I I charge-discharge;cycles. It was found thatthe discharge I I I I I i 3 I f I I I I I at fairly constant voltage; Because of this, cells emiploy- I (potential against time) of such I cathode-depolarized I I I I I I ing this mixed 'catliode -depolarizer yield almost all the I I I I electrodes was the same after 40 cycles as it was upon I I I I I I outputtheoretically potentially in the cell I I I I I I I I I I I initial discharge. Negative electrodes used in the cells I I I I I I I I I I I The following may be cited as'specificexarnples of the II I practice ofthis' invention? I I I 1 I I I I thus tested vwere made 'of'zincand cadmium ,but i has I I I been found that many other anodes. are adapted to use in' I I I conjunction with the cathode-dcpolarizer electrode described herein.

what isclaimedi sz I. I I 1 I The. method of ,forming a cathode-depolarizer mix I I I I for rechargeable. alkaline galvanic cells {which comprises. I I I I I I I I I I I I powdered mercuric oxide, throughZSQrnesh .(Tyler'Stand-q I I adding finely. divid d. mercuric. oxide powder toI a' clear I I I I I I I i I I I I I I I I fatal); to 247.02 grams of; a 50 percent by weightaqueous I aqueousisolution of manganese nitrateymixing the slurry} 1 II I I I I l n O l i i fiitrfltI- slurry thus formed I 1 I mus formed; drying said slurry to. a da p m s ena I 1 I E I I was thoroughly rni x'ed and then dried under constant 0 ting said mass intopellets; and bakingsaid vpelletsin an. I I I fagitationc The damp massthus obtainedwas then broken. I ;ov en I I I I I I I I I I I III I I I I I I I I I I I into pellets about 0.0098 in'chin diameter, and the pellets 1 I I 2.; The method ff min I cathgjde depolayizyep I I I I I j I I j I i I 'I 'I 1 h ated: injanoven at 500 F. for i3O mi nutes.- After this I for rechargeable alkaline: galvanic cells which comprises I I I I I I I I II I I the I pellets were ground into particles I small enough to II adding 250 mesh (Tyler Standard) mercuric oxide powder I; I I I I I I I I I I pass through I a 200 mesh (Tyler Standard) screen and to a clear aqueous solution of. manganesg it t i i I I I I I I I I I I I I stored for future use: in rechargeahle alkaline cells; 1 I I I the slurry thus formed; dryingisaid slurryto a damp mass I I I I I I I I I I I I I I I II I I while mixing'said slurry; separating said mass into pellets; I I I I I I I I and baking said pellets in an oven. at about .500 F. for.

I i I A cathode-depolarizer mixcontaining 83.2 percent by 1 about thirty. minutes I I I II I I Weight Im'fifcllfifi Oxide 3116163 P by Weigh? man'i '3. A rechargeable:cathode-depolarizer mix. for: in I I I I I I I I 8 1 1 X d w P P d l/ adding 30 of v I alkaline galvanic; cells I comp-rising particles, of mercuric. I I l l i j mesh (Tyler: a d d) mercuric oxlde to 274 grams of a I I l oxide each or which is substantially: coveredzliy a hard I Z I I I I I percent by weight aqueous solution- :of manganese I I shell of'manganese di id I I I I I I I I I I I I I I I I I nitrate. i The siurryithus formed was hfl m g ly' m x d I I I ,4 A ca hode-,depolarizer mix for rechargeableialkaline I I I I I I I I I I 15811 dried under cflnstant agitation Th6 p mass. 35. galvanic I cells which comprises; ai mass. of individual I I I I Example I I I I I A cathode-depolarizer mix containing 69 percent by:

I I I I weight mercurieoxide. and 31 percent by weight man- 'g'anese dioxide was prepared by adding 150 grams of I Example 11,

cells.

Example 111 A cathode-depolan'zer mix containing 25 percent by weight mercuric oxide and percent by weight manganese dioxide was prepared by adding 14 grams of 250 mesh (Tyler Standard) mercuric oxide to 172 grams of a 50 percent by weight aqueous solution of manganese thus obtained was then: broken into pelletssma l: eno g I I I I I I I topassthrough a fl'rnesht'fyler Standard) screen, and; I I

' I I I I I I 3 3 I I the pellets heated in an oven atSQO" :F. for '30 minutes; I I I I I II I After this the pellets: were ground: into particles small. I

I enough to pass I through l a 200 mesh :(Tyler Standard) screen and stored for future use in rechargeable alkaline I particles of mercuricoxidewherein each individual. parti II I I clei is substantially completely; coated with manganese I I I 1 dioxide: said mercuric oxide consstituting 5 percent to 87 I I I percentby weightofsaid mix. I I I I I I I :5. A cathodedepolarizer mix-for rechargeable; alkaline I I i galvanic cells which comprises a mass-of individual parti- I I I cles of mercuric oxide wherein each individual particle is substantially completely coated with manganese dioxide, said mercuric oxide constituting 52 percent to 87 percent by weight of said mix.

References Cited in the file of this patent UNITED STATES PATENTS nitrate. The slurry thus formed was thoroughly mixed and then dried under constant agitation. The damp mass 50 1,160,999 K plan NOV. 16, 1915 thus obtained was then broken into pellets less than 0.01 1,195,677 Heil Aug. 22, 1916 inch in diameter, and the pellets heated in an oven at 1,434,469 Wilker Nov. 7, 1922 500 F. for 30 minutes. After this the pellets were ground 2,481,539 Ruben Sept. 13, 1949 

